1. Field of the Invention
The present invention relates to an image sensing system and a method of driving the image sensing system.
2. Description of the Related Art
Known image sensing systems, such as a digital camera and a camcorder, include an image sensing system that performs auto-focusing (AF) based on phase difference detection using sensors for image sensing.
Japanese Patent Laid-Open No. 2000-156823 discloses a solid-state image sensing apparatus configured such that a plurality of photoelectric conversion cells for obtaining ranging signals are arranged in some portions of a pixel array included in the apparatus. The photoelectric conversion cells include a pair of photoelectric conversion cells each having an opening which is offset relative to the center of the cell, the openings being offset in opposite directions away from each other.
As disclosed in Japanese Patent Laid-Open No. 2000-156823, the solid-state image sensing apparatus has a skipping mode and a ranging mode as operation modes. In the skipping mode, signals are read from the photoelectric conversion cells while the photoelectric conversion cells in the portions are skipped, since signals from the photoelectric conversion cells for ranging in the pixel array are not used for image generation. Movie or view finder image data is generated based on data obtained in the skipping mode. Skipping readout enables high-speed photometry. In the ranging mode, signals are read from lines including the photoelectric conversion cells for AF.
However, exposure control may not be properly performed during operation disclosed in Japanese Patent Laid-Open No. 2000-156823. To perform exposure control and image generation in the skipping mode, signals are read from the required number of photoelectric conversion cells for an image. Accordingly, the time required to obtain signals of one frame becomes longer. Under conditions that the quantity of light from an object considerably varies due to, for example, a strobe in a short time, the exposure control may not be able to follow the change of light quantity.